In military combat helmets different types of interior structures may be used i.e. various strap, cushion, foam upholstering and dot systems. The interior structures fulfil—among others—the function of fit, stability and impact absorption.
DE 921 777 describes a helmet with a cranium strap that runs around the user's cranium. The helmet shell is connected to the cranium strap via intermediate elements that each comprises an elastic material and two metal plates vulcanized to opposite sides of the elastic material. Rivets connect the metal plates to the helmet shell and cranium strap respectively. The metal plates and the rivets function to attach the helmet and cranium strap to the elastic material, which is vulcanized to the metal plates. When an object impacts on the helmet shell, the metal plates transfer to the impact force to the elastic material, thereby compressing the elastic material so that part of the impact energy is absorbed.
The helmet of a combat soldier primarily protects the head against ballistic threats (bullets and fragments) and against “blunt impact” (falling/bumping). Besides, the helmet more and more is used as a platform to which parts can be attached, like sensors, interfaces and other peripheral equipment. For the performance and security of the user it is important that the helmet is seated on the user's head in a comfortable and stable way. The interior structure of the helmet plays an important role in the helmet's fit: it forms the interface between the helmet shell and the user's head. In the interior structure many functions are combined:                fitting to the user's head size and shape by adaptation of the interior structure;        absorbing impact of energy in case of falling and bumping;        stabilizing the helmet relative to the head by minimalisation of the motions (rotations and translations);        spacing the helmet shell from the cranium in order to allow indentation in case of impact of a projectile (sometimes called “stand-off”);        ventilation by providing free air flow in order to drain warm, humid air;        force transfer by transferring the mass of the helmet to the cranium        
Some requirements conflict one another: a helmet has to be stable on the head, which means that a helmet is hardly allowed to rotate and translate relative to the head; for impact absorption, however, it is necessary that the helmet shell is connected to the interior structure independently of the head, allowing it to move relative to the head. For a good fit and wear comfort it is necessary that the interior structure is adjusted so that it fits well to the shape and size of the head, where the interior structure has to be secured after being adjusted so that the size setting remains unaltered during use of the helmet. This setting, on one hand, has to be rigid enough to create a stable helmet and, on the other hand, flexible enough to be dented on impact.
In the helmet of DE 921 777 the blocks of elastic material absorb the impact energy. The metal plates on opposite sides of the elastic material are used to transmit the forces to the elastic material. The metal plates are only coupled to each other via the elastic material.
The helmet of DE 921 777 has the disadvantage that it is difficult to adapt the fit of the helmet to the user's head. Different size helmets must be provided for different users, or helmets with intermediate elements with different size blocks of elastic material would have to be provided, riveted to the helmet shell and the cranium strap.